Background: Existing research has predominately focused on dyadic relationships in families of children with intellectual disabilities. The aim of this study was to build on emerging literature exploring triadic relationships between a mother, sibling, and child with intellectual disability, investigating how they influence each other's well-being. Method: An online survey was used to collect information regarding the mother's mental health and the emotional and behavioural problems of the sibling and the child with intellectual disability in 573 families. Results: Using structural equation modelling, we found that maternal psychological distress was associated with higher levels of behaviour problems in the sibling, and the behaviour problems of the child with intellectual disability were associated with higher levels of maternal distress. Conclusions: Family member well-being is inter-related in families of children with intellectual disabilities. Clinical interventions that improve the behaviours of children with intellectual disabilities should be considered.